I have three suggestions for you.
The three awesome chapter packs.
The issue with the chapter packs is that they are designed for all houses. Brotherhood tried something where each pack was skewed towards one House, but the interactions of the cards contained in the Brotherhood cycle are extremely co-dependant: for example, you simply couldn't build a working House Bolton deck without owning all 6 Brotherhood packs. So steer away from those.
In all honesty, you're best off just going for the chapter packs that are fairly equitable for all 6 houses. In this, there are three chapter packs which are considered to be the Gold Standard.
Ancient Enemies from the Clash of Arms cycle gives you the awesome Fury plots, which help to cement the alliances established in the books. Each plot is directed to the two houses which are most at odds with the Fury plot's owner. In addition, they grant you 5 gold and 7 initiative, which can make for some real game-changing turns.
Sacred Bonds, also from Clash of Arms, compliments the Fury plots by playing off their traits (Battle/Gambit/Struggle), as each House gains a powerful event card that allows you to both stand a character and trigger some effect, on the proviso that you have played your Fury in a previous turn.
The final chapter pack which is pretty much an auto-buy is Refugees of War. It gives each House 3 refugees: 0 cost 2 str double icon characters. They generally only last a turn, but they allow you to field a larger force most of the time. The utility of this chapter pack cannot be understated.
The only caveat I would add here is that these chapter packs may, in the not-too-distant future, get reprints (where they are upgraded from the 40-card model to the 60 card model). This would especially benefit the Sarced Bonds pack as there is only one copy of each To Be A ____ events, when really you could do with 3. If you're really looking to get the best deal as opposed to the best utility, might want to look elsewhere.
Another core set.
It probably costs no more than three chapter packs, and the utility of another core set is perhaps unparalleled in your early AGoT experience. It allows you start switching up your plots which makes the games a lot more varied, and you can start aiming at the 60-card minimum required for standard play. You also get saves on your key uniques and a good set of resource-locations to play about with. There's something in it for each House as well: Starks benefit from more Dire Wolves and Kennel Masters, Baratheon's get more of those annoying attachments and Targ's pretty much need all the Forever Burnings they can get.
If you join agotcards.org and browse through the Cards page there, it has links to all the different Chapter packs and a rating for each card contained therein. That might give you some indication as to what packs are particularly useful for your specific Houses, but make sure to read the rules text carefully on each card as they might be highly dependant on some other combo of cards which will cost you a lot more to obtain.